Retention and drainage is an important aspect of papermaking. It is known that certain materials can provide improved retention and/or drainage properties in the production of paper and paperboard.
The making of cellulosic fiber sheets, particularly paper and paperboard, includes the following: 1) producing an aqueous slurry of cellulosic fiber which may also contain inorganic mineral extenders or pigments; 2) depositing this slurry on a moving papermaking wire or fabric; and 3) forming a sheet from the solid components of the slurry by draining the water.
The foregoing is followed by pressing and drying the sheet to further remove water. Organic and inorganic chemicals are often added to the slurry prior to the sheet-forming step to make papermaking less costly, more rapid, and/or to attain specific properties in the final paper product.
The paper industry continuously strives to improve paper quality, increase productivity, and reduce manufacturing costs. Chemicals are often added to the fibrous slurry before it reaches the papermaking wire or fabric to improve drainage/dewatering and solids retention; these chemicals are called retention and/or drainage aids.
Drainage or dewatering of the fibrous slurry on the papermaking wire or fabric is often the limiting step in achieving faster paper machine speeds. Improved dewatering can also result in a drier sheet in the press and dryer sections, resulting in reduced energy consumption. In addition, as this is the stage in the papermaking method that determines many of the sheet final properties, the retention and drainage aid can impact performance attributes of the final paper sheet.
With respect to solids, papermaking retention aids are used to increase the retention of fine furnish solids in the web during the turbulent method of draining and forming the paper web. Without adequate retention of the fine solids, they are either lost to the mill effluent or accumulate to high levels in the recirculating white water loop, potentially causing deposit buildup. Additionally, insufficient retention increases the papermakers' costs due to loss of additives intended to be adsorbed on the fiber. Additives can provide opacity, strength, sizing or other desirable properties to the paper.
High molecular weight (MW) water-soluble polymers with either cationic or anionic charge have traditionally been used as retention and drainage aids. The efficacy of the polymers or copolymers used will vary depending upon the type of monomers from which they are composed, the arrangement of the monomers in the polymer matrix, the molecular weight of the synthesized molecule, and the method of preparation.
Recent development of inorganic microparticles, when used as retention and drainage aids in combination with high MW water-soluble polymers, have shown superior retention and drainage performance compared to conventional high MW water-soluble polymers. U.S. Pat. Nos. 4,294,885 and 4,388,150 teach the use of starch polymers with colloidal silica. U.S. Pat. Nos. 4,643,801 and 4,750,974 teach the use of a coacervate binder of cationic starch, colloidal silica, and anionic polymer. U.S. Pat. No. 4,753,710 teaches flocculating the pulp furnish with a high MW cationic flocculant, inducing shear to the flocculated furnish, and then introducing bentonite clay to the furnish.
It had been found recently that water-soluble copolymers prepared under certain conditions exhibit unique physical characteristics. These polymers are prepared without chemical cross-linking agents. Additionally, the copolymers provide unanticipated activity in certain applications, including papermaking applications, such as retention and drainage aids. The anionic copolymers that exhibit the unique characteristics were disclosed in WO 03/050152 A1, the entire content of which is herein incorporated by reference. The cationic and amphoteric copolymers which exhibit the unique characteristics were disclosed in U.S. patent application publication 2004/0143039 A1, the entire content of which is herein incorporated by reference.
The use of inorganic particles with linear copolymers of acrylamide, is known in the art. Recent patents teach the use of these inorganic particles with water-soluble anionic polymers (U.S. Pat. No. 6,454,902) or specific crosslinked materials (U.S. Pat. Nos. 6,454,902, 6,524,439 and 6,616,806). However, there still exists a need to improve drainage and retention performance.